What is the best place to grind for mush in Explore?

I would in forest 2, for BE training/ Mush grinding, and I've been playing for one year now.

Thank you! I also heard jungle and tundra were good places to go?

Forest 2 is not nearly as profitable as jungle or tundra in terms of mush - low level forest is mainly good for BE training. As long as your wolves are good explorers, jungle and tundra should net you the most mush due to their high predator and prey counts.